Three out of four advance tests the oil services firm Halliburton carried out on a cement mixture used to try to close BP's Macondo oil well in the Gulf of Mexico showed that the cement would be unstable, according to evidence collected by the National Oil Spill Commission.

Halliburton told BP about only one of those tests before the April 20 well explosion - the data indicated the cement plug would probably not hold, yet BP and Halliburton went ahead with the cementing job, according to the commission.

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